1. Field of the Invention
The present invention relates to a solid state radiation detector having a storage section for storing an amount of electric charges as latent image charges that corresponds to the amount of radiation irradiated or light emitted through excitation by the radiation.
2. Description of the Related Art
Today, various types of radiation image information recording/reading systems that employ a solid state radiation detector (hereinafter also referred to as simply “detector”) are proposed in the field of radiation imaging for medical diagnosis. The detector temporarily stores in its storage section electric charges as latent image charges obtained by detecting radiation, and outputs electrical signals by converting the latent image charges. Various types of detectors are proposed as the solid state detector for use in the system. From the aspect of charge readout process for reading out charges stored in the detector, the detectors may be classified into one of the types called the optical reading type in which the charges are read out by emitting reading light (reading electromagnetic wave) on the detector.
Such type of detector is proposed by the applicant as the optical reading type solid state radiation detector having high reading response and efficient signal charge extraction capabilities, as described in Japanese Unexamined Patent Publication No. 2000-105297 and U.S. Pat. Nos. 6,770,901 and 6,518,575. The detector has a set of layers layered in the order of: a planar first conductive layer which is transparent to recording radiation or light emitted through excitation by the recording radiation (hereinafter referred to as “recording light”); a recording photoconductive layer that shows conductivity when exposed to recording light; a charge transport layer that acts as substantially an insulator against charges having the same polarity as the charges charged on the first conductive layer and as substantially a conductor for the charges having the opposite polarity; a reading photoconductive layer that shows conductivity when exposed to reading light; a second conductive layer which is transparent to reading light. The layer composite has a storage section formed between the recording photoconductive layer and charge transport layer for storing latent image charges (electrostatic latent image) carrying image information.
The solid state radiation detector proposed by the applicant in U.S. Pat. Nos. 6,770,901 and 6,518,575, in particular, uses a striped electrode composed of multitudes of charge detecting linear electrodes which are transparent to reading light as the electrode of the second conductive layer having transparency to reading light. In addition, the detector further has multitudes of auxiliary electrodes for outputting an amount of electrical signals corresponding to the amount of latent image charges stored in the storage section. The auxiliary electrodes are opaque to reading light and installed in the second conductive layer such that they are disposed alternately and substantially parallel to the charge detecting linear electrodes.
By providing a sub-striped electrode made of the multitudes of auxiliary linear electrodes in the second conductive layer, a capacitor is newly formed between the storage section and sub-striped electrode. This allows the transport charges having opposite polarity to the latent image charges stored in the storage section by the recording light to be charged also on the sub-striped electrode through the charge rearrangement process at the time of reading. This may reduce the amount of transport charges to be allocated to the capacitor formed between the striped electrode and storage section through reading photoconductive layer compared with the case where no such sub-striped electrode is provided. Consequently, the amount of signal charges which may be extracted from the detector to the outside is increased and the reading efficiency is improved, resulting in high reading response and efficient signal charge extraction capabilities.
In the mean time, in the solid state radiation detector described above, a high voltage is applied between the planar first conductive layer and second conductive layer having the striped electrode and sub-striped electrode when latent image charges are recorded. At that time, the electric fields concentrate on the edge of the planar first conductive layer, so that it is difficult to evenly distribute the electric fields on the entire recording surface of the solid state radiation detector at the time of recording.
If the electric fields are unevenly distributed on the recording surface of the solid state radiation detector, artifacts may result. Currently, therefore, it is customary that a certain predetermined width extending inwardly from the outer edge of the first conductive layer is defined as the non-image-detection area, and only the further inner region is used as the image detection area in order to avoid the effects of the electric field concentration.
Further, for the solid state radiation detector described above, after an electrostatic latent image is recorded and read out from the detector, it is customary to irradiate erasing light to erase residual images remaining in the detector for the subsequent recording. The edge of the conductive layer receives highly concentrated electric fields at the time of recording, so that the region adjacent to the outer edge of the first conductive layer has more residual images than on the image detection area. Thus, the residual images in the region adjacent to the outer edge of the first conductive layer may remain even after the residual image erasing process is performed. Consequently, more residual images are accumulated as the detector is repeatedly used, which may invade the image detection area and cause artifacts to be developed even on the image detection area.
The solid state radiation detector having the sub-striped electrode for increasing reading efficiency, in particular, the phenomenon described above is significant, since erasing light is not incident on the region above the sub-striped electrode because of its opaqueness to light, and most part of the residual images formed on the region remains even after the residual image erasing process is performed.
Of course, the problem described above may be alleviated by broadening the non-image-detection area, but the solid state radiation detector having the widest possible image detection area to the size of the detector is desired. Thus, in considering the narrower trimming region, broadening the non-image-detection area is unrealistic.